This invention relates to anaerobic treatment of wastewater. For purposes of this invention the term "wastewater" includes organic municipal, industrial or commercial waste.
Conventional systems for treating wastewater employ a settling tank to assist in separating the liquid from the solids. After a satisfactory holding time in the settling tank, sludge from the bottom portion is withdrawn and delivered to a digester where anaerobic microorganisms perform their known chemical decomposition.
Anaerobic digestion of organic solids is accomplished by two major groups of anaerobic microorganisms. The first group (facultative bacteria) liquefies and converts the complex structures of the organic sludge particles primarily into volatile organic acids (acetic, proprionic, butyric) with the release of carbon dioxide, water and some nitrogen sulfite. The second group of bacteria (methane formers) transforms the volatile organic acids into cell structures, stable degradation products, methane gas and carbon dioxide.
The methane forming bacteria are fragile as to temperature changes and pH. Where the sludge is allowed to cool or heat outside the temperature range in which the methane forming bacteria can survive, they will die. Similarly, where the volatile acids generated by the first phase of the digestion process become too concentrated, the methane forming bacteria will also die. Ether deviation will result in a halt in the desired process. Without good distribution or effective auxiliary stirring of the incoming feed sludge, the feed sludge is pumped into a point mass, thus, seriously overloading the bacterial food supply in that part of the digesting sludge, resulting in a preponderance of voltatile acid formation and an inhibition or total cessation of the methane phase of the digestion process. The absence of the methane stage, whether it be from temperature or pH change, results in an unstabilized sludge and the lost production of methane which is a valuable fuel gas. Thorough mixing of the incoming feed sludge with the mass of well diversified seeded digesting sludge is, therefore, of prime importance in producing methane and well stabilized sludge.
Some systems employ a heating system of the incoming sludge to maintain the proper temperature balance within the digester and various kinds of mixing apparatus have been employed. However, it has been discovered that vigorous agitation of the digesting sludge hydrolyzes and homogenizes the organic mass and liquid separation is inhibited.
Solids in the digestion tank or the settling tank are for the most part not greatly different in density than water itself and as a result, the gravitational separation of the solids from the liquid components takes considerable time. For example, in a settling tank complex, it may be several hours before the sludge will be sufficiently concentrated in the bottom of the tank to be withdrawn satisfactorily. It is no benefit to the system to withdraw large portions of liquid with the settled solids before delivering it to the digestion tank. It is intended that the liquid be separated by the gravitational process and be drawn off from the top of the settling tank for delivery elsewhere for subsequent treatment where necessary. Treatment of the liquid phase of the wastewater is not technically a part of this invention and conventional treating processes are to be employed with the liquid phase.